The present invention relates to process variable transmitters used in process control and monitoring systems. More specifically, the present invention relates to dynamically adjusting an input to an analog-to-digital converter in such a system, in order to improve the resolution of the A/D measurement, during operation.
Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 mA control loop, or wirelessly.
One exemplary parameter that is measured by such a system is temperature, which is sensed by measuring the resistance of a resistive temperature device (RTD, which is also sometimes called a platinum resistance thermometer or PRT) or the voltage output by a thermocouple. Of course, temperature is only exemplary, and a wide variety of other process control parameters can be measured as well.
Some typical A/D converters are configured with a fixed voltage reference and a configurable resolution gain adjustment. Each gain setting for the configurable resolution gain adjustment corresponds to a reference point that is a scalar of the attached voltage reference. The voltage reference is used by the A/D converter to set the resolution (in volts per count) of the A/D converter. The resolution of the A/D converter can thus be changed based on an expected signal level of the analog input signal being measured.
Conventional monitoring systems, in which A/D converters are implemented, often have the configurable resolution gain adjustment set so that the A/D converter can measure the full range of a specified sensor. That is, the configurable resolution gain adjustment is set for a single, fixed resolution which can accommodate the entire sensor range. However, the actual input signal may often have a value (or vary within a range of values) for which the A/D converter could be set to a much higher resolution. Thus, setting the gain adjustment to a set, fixed resolution results in a measurement circuit that is not tuned to use the full capability of the A/D converter.